A complex birefringent medium is an optical element capable of converting a polarized state (linearly polarized light, circularly polarized light, elliptically polarized light) of incident light. Such the birefringent media are industrially widely used as phase difference films typified by a λ/4 plate having a phase difference (birefringent phase difference, retardation) corresponding to ¼ of an incident light wavelength λ and a λ/2 plate having a phase difference corresponding to ½ of an incident light wavelength λ in liquid crystal display devices or the like together with linear polarizers capable of obtaining linearly polarized light from natural light. As a conventional phase difference film, thin plates composed of inorganic materials such as calcite, mica, crystal and the like and stretched polymer films formed by stretching a polymer film having large intrinsic birefringence (ultimate birefringence value) at high draw ratio to enhance a degree of orientation of a high polymer chain and to develop oriented birefringence are well known. Furthermore, recently, a phase difference film in which the orientation of liquid crystal is fixed or the like is known.
The λ/4 plate has an optical function of converting the linearly polarized light to the circularly polarized light, and it is applied for a circularly polarizing plate or the like. Since the circularly polarized light has a property in which chirality of right and left is replaced each other in reflecting the circularly polarized light with a mirror or the like, when a left-handed circularly polarizing plate is disposed on a mirror and light is irradiated, light passing through the left-handed circularly polarizing plate and converted to left-handed circularly polarized light is converted to right-handed circularly polarized light by reflection on the mirror, and this right-handed circularly polarized light cannot pass through the left-handed circularly polarizing plate. That is, the circularly polarizing plate has an optical function of reducing reflection. Such a reflection-protective optical function of the circularly polarizing plate is applied to for reducing reflection on an internal electrode in organic EL or the like. Further, the λ/2 plate has an optical function of converting the orientation (vibration direction) of the linearly polarized light. Furthermore, a phase difference film having a phase difference other than a quarter and a half of an incident light wavelength λ also has an optical function of converting the polarized state (ellipticity and/or orientation) of incident light to another state.
These phase difference films are generally designed so as to exhibit required optical functions to light of specified wavelength (monochromatic light). For example, the λ/4 plate is generally adjusted to a phase difference corresponding a quarter of a wavelength at only a specific design central wavelength, and it is not adjusted to a phase difference corresponding a quarter of a wavelength due to the wavelength dispersion of intrinsic birefringence resulting from the materials of the phase difference film at other wavelength. That is, in the conventional phase difference films, since it is common that an absolute value of the intrinsic birefringence, which the material exhibits, generally becomes larger as the wavelength is shorter and becomes smaller as the wavelength is longer, that is, exhibits normal wavelength dispersibility. If a phase difference is adjusted to, for example, 137.5 nm which is a quarter of 550 nm at 550 nm of a design central wavelength, a phase difference at a wavelength of 450 nm becomes, for example, 148.5 nm which is larger than 137.5 nm, and this is larger than 112.5 nm which is a quarter of 450 nm. Even if the phase difference at a wavelength of 450 nm is identical to the phase difference 137.5 nm at a wavelength of 550 nm, it is larger than 112.5 nm. Further, a phase difference at a wavelength of 650 nm becomes, for example, 132 nm which is smaller than 137.5 nm, and this is smaller than 162.5 nm which is a quarter of 650 nm. Even if the phase difference at a wavelength of 650 nm is identical to the phase difference 137.5 nm at a wavelength of 550 nm, it is smaller than 162.5 nm. White light is irradiated to such the phase difference film, output light is colored because converting of the polarized state varies from one wavelength to another.
In order to prevent such coloring, various investigations is made concerning a wideband phase difference film capable of providing an optimal phase difference for light in a wide range of visible wavelength range, specifically, a phase difference film having the so-called inverse wavelength dispersibility (inverse characteristics of normal wavelength dispersibility) in which an absolute value of a practical phase differences becomes smaller as the wavelength is shorter and an absolute value of a practical phase differences becomes larger as the wavelength is longer. As such the phase difference film having the inverse wavelength dispersibility, a phase difference film formed by laminating two stretched film having different wavelength dispersion of intrinsic birefringence in such a way that stretching directions (or optic axes, slow axes, fast axes) are perpendicular to each other is disclosed (for example, refer to Patent Documents 1 to 5).
Further, a phase difference film formed by bonding a λ/4 plate to a λ/2 plate with two stretching directions (or optic axes, slow axes, fast axes) crossed is disclosed (for example, refer to Patent Documents 6 and 7). Further, a phase difference film formed by laminating at least two phase difference films having a phase difference of 160 to 300 nm with two slow axes not parallel and not perpendicular to each other is disclosed (for example, refer to Patent Document 8). Furthermore, a method, in which a phase difference layer to use liquid crystalline molecules is used for at least one layer of two or more stretched films in place of a method of laminating two or more stretched films, is also investigated. For example, a phase difference plate, which is formed by laminating a first optically anisotropic layer containing liquid crystalline molecules and substantially having a phase difference of n and a second optically anisotropic layer containing liquid crystalline molecules and substantially having a phase difference of π/2 on a long transparent support in such a way that an angle between a in-plane slow axis of the second optically anisotropic layer and a in-plane slow axis of the first optically anisotropic layer is substantially 60 degrees, is disclosed (for example, refer to Patent Document 9). In addition, a phase difference film formed by laminating an optically anisotropic layer having a phase difference of 210 to 300 nm at a wavelength of 550 nm and an optically anisotropic layer having a phase difference of 115 to 150 nm at a wavelength of 550 nm, in which one optically anisotropic layer is made of a polymer film and the other optically anisotropic layer is made of a liquid crystalline molecule, is disclosed (for example, refer to Patent Document 10).
In contrast, a technique in which a wideband phase difference film is realized by one phase difference film is proposed (for example, refer to Patent Documents 11 and 12). In this method, a polymer film composed of a copolymer and/or a blend polymer containing a monomer unit having positive refractive index anisotropy and a monomer unit having negative birefringence is uniaxially-stretched. In accordance with this method, a wideband phase difference film can be realized by one layer phase difference film without laminating phase difference films and a viewing angle characteristic can be improved.
Further, a low-profile wideband phase difference film which can be produced by a simple production process is contemplated and a liquid crystal composition exhibiting inverse wavelength dispersibility is also proposed (for example, refer to Patent Documents 13). In this method, a liquid crystal layer containing a compound having two or more species of mesogenic groups and a rod-shaped liquid crystal compound is provided, and by aligning the rod-shaped liquid crystal compound so as to have homogeneous alignment, and aligning at least one species of the mesogenic group of the compound having mesogenic groups at a 45 to 90 degree angle with respect to an optic axis of the rod-shaped liquid crystal compound in a film-plane, the inverse wavelength dispersibility is tried to be developed.
Furthermore, a phase difference film having inverse wavelength dispersibility formed by aligning a liquid crystal monomer having discotic mesogen and nematic mesogen having a polymerizable group at its end in a molecule in such a way that an optic axis of discotic mesogen is substantially parallel to an optic axis of nematic mesogen is proposed (for example, refer to Patent Documents 14). Further, a method of laminating a stretched film having an optically-positive uniaxial property and a stretched film having an optically-negative uniaxial property with stretching directions parallel to each other is known (for example, refer to Patent Document 15).
[Patent Document 1]
Japanese Kokai Publication No. Hei-3-13916
[Patent Document 2]
Japanese Kokai Publication No. Hei-3-263013
[Patent Document 3]
Japanese Kokai Publication No. Hei-4-121703
[Patent Document 4]
Japanese Kokai Publication No. Hei-5-27119
[Patent Document 5]
Japanese Kokai Publication No. Hei-10-239518
[Patent Document 6]
Japanese Kokai Publication No. Hei-10-68816
[Patent Document 7]
Japanese Kokai Publication No. Hei-5-100114
[Patent Document 8]
Japanese Kokai Publication No. Hei-10-90521
[Patent Document 9]
Japanese Kokai Publication No. 2001-4837
[Patent Document 10]
Japanese Kokai Publication No. 2000-284126
[Patent Document 11]
WO 00/26705
[Patent Document 12]
Japanese Kokai Publication No. 2003-207640
[Patent Document 13]
Japanese Kokai Publication No. 2002-267838
[Patent Document 14]
Japanese Kokai Publication No. 2005-208414
[Patent Document 15]
Japanese Kokai Publication No. Hei-3-13917